1. Field of the Invention
The present invention relates to switching voltage regulators, and in particular to a switching voltage regulator wherein a transistor supplying current to a load is rendered conductive in a selectively controlled manner to minimize RFI (radio frequency interference) noise generated by a flyback diode.
2. Description of the Prior Art
Conventional prior art switching voltage regulators convert an unregulated voltage applied across an input and a reference terminal to a regulated voltage across a load between an output terminal and the reference terminal. In one common type of voltage regulator, a switching transistor is connected in series with an induction coil between the input and the output terminals, and a diode, poled to be nonconductive when the switching transistor is conductive, is connected between the reference terminal and the juncture between the switching transistor and the coil.
In the operation of this type of voltage regulator, the switching transistor is switched between its saturated and its cutoff states in response to variations in the value of the regulated voltage between the output and the reference terminals. More particularly, the transistor is maintained in saturation to reverse bias the diode and to establish a current path between the input and the output terminals, and through the induction coil to establish a magnetic field therewith, until the regulated voltage across the load between the output and the reference terminal increases to a first, or maximum, value. When the regulated voltage reaches the first value, the transistor is rapidly switched from its saturated to its cutoff state. This induces a voltage in the coil, as a result of the interruption of the current flow between the input and the output terminals, which forward biases the diode to establish a current path between the reference and the output terminals and through the diode and the coil. The collapsing field of the coil maintains the current flow between the reference and the output terminals until the regulated voltage across the load decreases to a second, or minimum, value. At this point the transistor is rapidly switched to its saturated state which rapidly reverse biases the diode, and the current path is again established between the input and the output terminals, and through the transistor and the coil, until the regulated voltage across the load again reaches the first value, at which point the described cycle of operation is repeated.
A disadvantage of this type of voltage regulator is that the switching transistor is very rapidly switched from its cutoff to its saturated state, which very rapidly reverse biases the diode. When the diode is very rapidly reverse biased, or "snapped off", wide band RFI noise is generated thereby. As a result of the RFI noise generated by the diode, when this type of conventional regulator is used in, for example, power supplies in mobile radio and microwave transmission equipment, on which rigid frequency limitations are imposed, extensive RFI shielding must be employed to prevent the transmission of the RFI noise therefrom, which shielding adds substantial cost to the equipment. Also, when the conventional regulator is employed in integrated circuits, the RFI noise generated thereby may interfere with logic functions.
Another disadvantage of this type of switching voltage regulator is that conduction of the switching transistor is controlled by fluctuations in the value of the regulated voltage. Accordingly, ripple exists in the regulated output voltage.
Furthermore, with this type of conventional switching voltage regulator the danger exists, during power turn on or in the event that a short or overload condition develops at the output thereof, that circuit components may be destroyed as a result of large power dissipations caused by excessively large currents conducted by the switching transistor in attempting to bring the voltage across the reference and the output terminals to the first value. For example, if a short develops across the load at the output of the regulator, the switching transistor will not be able, when in its saturated state, to increase the voltage across the output terminals to the first value, and the transistor will continue to conduct heavy currents which could result in the destruction of the transistor or of other components. Similarly, when power is initially applied to the voltage regulator, heavy surge currents demanded by a load (i.e., a capacitive load) could result in stressing of circuit components of the regulator.